Method and apparatus for the transmision of an acoustic signal in a photoacoustic cell

Abstract

The microphone in a photoacoustic cell that is part of an apparatus for photoacoustic analysis of at least one substance is protected against the harmful effects from the analyzed substance by sonically communicating the microphone with the excitation zone through a waveguide that is at least partly located within the excitation zone. The substance to be analyzed is supplied to the excitation zone, in which zone an acoustic signal is generated by an acoustic frequency modulation of an optical signal directed into the substance in the excitation zone. The substance is prevented from reaching the microphone by supplying a protecting gas to a portion of the waveguide proximate to the microphone. The waveguide is an acoustic resonator in which at least two antinodes of the acoustic signal are present, the resonator being excited in one antinode by the acoustic signal and the microphone being mounted in another antinode such that the waveguide transmits the generated acoustic signal from the substance through the protecting gas to the microphone without any significant weakening of the acoustic signal.

We claim: 
     
       1. A method for transferring an acoustic signal to a microphone in a photoacoustic cell that is part of an apparatus for photoacoustic analysis of at least one gaseous substance and in which apparatus the microphone is protected against the harmful effects from the gaseous substance analyzed, comprising the steps of placing the microphone for receiving the acoustic signal from an excitation zone, which is located in an environment which is damaging to the microphone, outside of the damaging environment, sonically communicating the microphone with the excitation zone through a waveguide that is at least partly located within the excitation zone, supplying the gaseous substance to be analyzed to the excitation zone, in which zone an acoustic signal is generated by an acoustic frequency modulation of an optical signal directed into the gaseous substance in the excitation zone, preventing the gaseous substance from reaching the microphone by supplying a protecting gas to a portion of the waveguide proximate to the microphone, the waveguide being an acoustic resonator in which at least two antinodes of the acoustic signal are present in the waveguide, the resonator being excited in one antinode and the microphone being mounted in another antinode such that the waveguide transmits the generated acoustic signal from the gaseous substance through the protecting gas to the microphone without any significant weakening of the acoustic signal. 
     
     
       2. A method according to claim 1, wherein the protecting gas supplied to the waveguide is non-absorbing with respect to the optical signal and thus does not produce sound under illumination by the optical signal, is harmless to the microphone under the physical conditions prevailing in the environment in which the microphone is located and is chemically neutral to the gaseous substance under the conditions prevailing in the excitation zone. 
     
     
       3. A method according to claim 1, and further comprising the step of conducting the gaseous substance into a portion of the waveguide remote from the microphone, said remote portion of the waveguide constituting at least part of the excitation zone, and removing the gaseous substance and the protecting gas from within the waveguide through an outlet from the waveguide located between the excitation zone and the microphone. 
     
     
       4. A photoacoustic measuring cell comprising a housing (21) having a window (11) adapted to admit into the housing pulsating laser light and a microphone unit (10), a vessel remote from the housing and having a mirror (15) for reflecting the laser light mounted therein, an inlet (14) into the vessel for admitting a gaseous substance to be analyzed which is damaging to the microphone unit (10) and a waveguide (20) connected between the housing (21) and the vessel (22), the waveguide (20) having a length such that it constitutes a half wave acoustic resonator with respect to an acoustic signal generated by photoacoustic modulation by the gaseous substance of the laser light and allows the microphone (10) to be located outside the damaging range of the gaseous substance, a portion of the waveguide (20) proximate to the vessel defining an excitation zone (19) for analyzing the gaseous substance, an inlet (12) into the waveguide proximate to the microphone for admitting protecting gas (70) into the waveguide, and outlet (13) from the waveguide (20) at the end of the excitation zone nearer the microphone for discharging the gaseous substance and the protecting gas. 
     
     
       5. An apparatus according to claim 4, wherein the resonance frequency of the half wave resonator corresponds to the modulating frequency of the laser light or to a multiple of the modulating frequency. 
     
     
       6. An apparatus according to claim 4, wherein the prime tone of the half wave resonator has an antinode at each end of the waveguide. 
     
     
       7. An apparatus according to claim 4, wherein the frequency of resonance of the waveguide differs from the frequency of the photoacoustically generated acoustic signal of the gaseous substance being analyzed. 
     
     
       8. An apparatus according to claim 4, wherein the cell has two or more waveguides symmetrically mounted in relation to the excitation zone (30).